Electrode for piezoelectric crystal oscillators



June 29, 1943.

J. GRUETZMACHER ELECTRODE FOR PIEZOELECTRIC CRYSTAL OSCILLATORS Filed June 19, 1940 Patented June 29, 1943 ELECTRODE FOR PIEZ OSCILLA OELECTRIC CRYSTAL TORS Johannes Gruetzmacher, Berlin, Germany; vested in the Alien Property Custodian Application June 19, 1940, Serial No. 341,302 In Germany July 11, 1939 11 Claims. (Cl. 171-327) The invention relates to a new piezo-electric crystal oscillator for producing oscillations in liquids and is particularly concemed with an oscillator provided with a very sturdy and reliable electrode structure.

Oscillators of this kind usually comprise a plate-shaped crystal having on the two sides, that is, On the vibrating surfaces, plate-shaped electrodes. If an electrical alternating potential is connected to these electrodes, the frequency of which corresponds to the natural frequency of the oscillator, then the two vibrating surfaces of the crystal vibrate together with the crystal and with the two electrodes. The frequency of these oscillations, therefore, depends on the natural frequency of the oscillator; the smaller the oscillator is, the higher its frequency will be. The frequency may be within the range of the soundor ultra-sound oscillations.

Such piezo-electric crystal oscillators are used, for example, for the purpose of keeping constant the frequency in electrical circuits. The crystal oscillators can be driven with a very small output. It is, however, also known to utilize the oscillations of the crystal oscillator for treatment of some materials. For example, if the crystal oscillator is put into an oil-filled container, the oscillations are transmitted to the oil, whereby the oil can be de-gassed. The use of piezoelectric crystals is also known for the treatment of many other materials, for example, for sterilizing liquids.

Inasmuch as the crystal oscillator should operate in the treatment of liquids or other materials with as great an output as possible, such oscillators are put under considerable mechanical and electrical stresses, resulting in difliculties, particularly with respect to the electrodes. The electrodes are usually made very thin in order to transmit to the materials to be treated the vibrating energy as efiiciently as possible. Such thin electrodes are usually produced in such a manner that the crystal on its two sides is covered with a very thin film of gold or silver. These thin electrodes, however, have a very short life because they swing with the crystal and are mechanically put under great stress. They are also easily destroyed by the necessarily high electrical potential, because the high tensions lead to arcing. These conditions caused, among others, the following difficulties in the manufacture and use of such oscillators: if the electrode is made very thin, then its life is very short; if it is made thick, then the action of the oscillator is bad.

A piezo-electric crystal oscillator is already known in which the electrodes are made each of a thick plate or disk-shaped ring. Such electrodes have a. greater life than the thin electrodes, and the large opening of the ring facilitates the emanation of the vibrations, but these electrodes have the disadvantage that they do not sufliciently conduct to the vibrating surfaces the electrical energy which is necessary for the operation of the crystal. The sound-radiating surface of the crystal is also very much diminished by the plate-shaped ring. The efliciency and output of such oscillator are not very great.

The invention obviates the above mentioned difliculties by a particular structure of the electrodes. In the new oscillator at least one of the ,two electrodes is made of a'grid attached to the crystal and at least one of the two vibrating surfaces of the crystal is in contact with the liquid that is to be treated. The following advantages result from such a structure of electrodes:

1. The electrodes can be made thick so that their life will be great; and

2. The liquid which is to be treated is in direct contact with the crystal through the medium of the holes in the grid so that the oscillations of the crystal can be transmitted to the liquid unhindered, resulting in a very good efiect. The new oscillator therefore has the advantages that its electrodes are of practically unlimited life and permitting substantially constant operation even with great loads.

The size of the openings of the grid-shaped electrode, according to the invention, can be chosen as desired. For example, it can be made equal to half of the wave length of the oscillations, and it can also be larger or smaller. The larger the holes, the better the oscillations of the crystals will be able to emanate, but the conduction of electrical potential to the vibrating surfaces of the crystal is thereby made non-uniform. Co-ntrariwise, the smaller the holes, the greater the damping of the vibrations in the holes. Inasmuch as this damping is at a certain size of the holes smaller for small wave length than for great wave length, it is possible to make the holes smaller, the smaller the wave length is. Therefore, inasmuch as the efiEect of the crystal oscillator, according to the invention. and as far as the conduction of the tension to the crystal is concerned, is the better the smaller the holes are chosen, and on the other hand, with regard to the damping, the greater the holes are chosen. the manufacturer has it entirely in his hands to Figs. 4 and 5 represent another embodiment of the invention in section and in plan view, respectively;

Fig. 6 shows in fractional sectional view an embodiment having alternative fastening means; and

Fig. 7 illustrates a crystal of curved shape and electrodes similarly curved attached thereto.

Referring'now to the drawing, i denotes the piezo-electric crystal, both oscillating surfaces 'thereof being each provided with an electrode 2 and3 respectively. The electrodes 2 and 3 comprise a grid and stifiening rings 4 and 5, the meshes of the grid in the embodiment shown being of rectangular form. At the lower side of the crystal a corrugated diaphragm 6 is secured to the ring 5 so that a closed air space is provided between the diaphragm 6 and the crystal I. This air space hinders the crystal on its under side in the radiation of its energy, so that it produces sound waves propagating substantially in the upward direction as indicated by the arrows I.

'The length of mesh a: is chosen in accordance with the wave length so great that the sound when produced by the crystal may propagate through the meshes. The thickness y of the mesh walls is preferably chosen in accordance with the wave length so that the sonic or supersonic waves are deflected at the walls of the meshes. The thickness is, as a rule, so small that a resonance of the mesh walls is avoided. The height 2 of the grid 2 amounts preferably to one-half of the wave length (M2) or to an integral multiple thereof so that the portion of waves taken up by the grid may also propagate to the greatest possible extent. The height 2' of the grid 3, i. e., the height of the grid at the lower side at which the crystal is hindered in the radiation of its energy amounts preferably to one-fourth of the wave length (ll/4) or an integral multiple thereof. If both sides of the crystal, are to be set into oscillation, the height of the grid 3 is dimensioned in the same manner the height of the grid 2, as shown in Fig. 3,

wherein numerals I to 5 designate like parts shown in Fig. l, the arrows I indicating the sound waves-radiating from both sides of the crystal.

In the case of large outputs, parts of the electrode, for instance, the rings 4 and 5 may be provided with passages ll, as indicated in Fig. 3 to permit a cooling liquid to flow therethrough.

The shape of the electrode according to the invention is adaped to the shape of the crystal so that if the crystal is, for instance, curved the electrode is also curved. Such a structure is shown in Fig. 7, the crystal being indicated by numeral i and the electrodes by the numerals 4 and 5.

The electrodes may be secured to the crystal by means of insulating threads, for instance, silk threads which unite the two electrodes with one another at the edge of the crystal, as shown in Fi 3, wherein the silk threads are indicated at ll. They are attached to eyelets or loops II, as

shown, and the latter are soldered to the electrodes at the edges thereof. The electrodes may,

however, also be yieldably pressed against the crystal with the aid of springs l3, as shown in Fig. 6, insulating material I! being interposed between the electrodes and springs and the springs being fastened to a cylindrical holder, as shown, the holder being made of insulating material.

The electrode may also be made of a very thin plate firmly secured to the crystal by means of a binder. However, care should only be taken to see that it is not made too thin, i. e., as thin as a foil, so as to avoid the previously mentioned troubles attending the use of film or foil electrodes. Experience has shown that a thickness of about 0.5 to 1.5 mm. is advantageous depending upon the wave length. This electrode may be made in a simple manner, for instance, by punching, annealing, facing. grinding and then securing it .to the crystal. This type of electrode is particularly suitable in the case of small outputs for continuous operation notwithstanding the i'act that it is very simple in construction. If it is desirable to stiffen the thin electrode, stiifening ribs may be arranged on the electrode.

An oscillator provided with such an electrode is shown, in Figs. 4 and 5. The piezo-electric crystal is denoted by the numeral I. The two electrodes 2 and 3 consist of thin metal plates of the height a which are designed in the form of grids by punching therein meshes 8. To stiffen the electrodes they are surrounded by rings 4 and 5. Furthermore, stiffening ribs 9 are arranged on the electrodes forming thereon the grid structure, as shown. The height 2 which is equal to the height of the ribs or grid members and that of the electrode plate is so chosen that it amounts approximately to 7\/2 or to an integral multiple'thereoi'. However, it is sufficient that this height be maintained at least approximately, since the resonance range is rather wide owing to the damping in the metal. The distribution of the ribs 9 over the surface of the electrode may be effected in any suitable manner. In the embodiment shown in Figs. 4 and 5 the distribution is such that one or more meshes are arranged in each section. Experience has shown that a very effective stifi'ening and tuning of the electrodes is obtained with such arrangement.

The electrodes shown in Figs. 4 and 5 may be easily adapted to the form of the crystal, for instance, to the form of a curved crystal, as shown in Fig. 7. Numerals I to 5 designate in this figure parts referred to by like reference numerals appearing in Fig. 1. The electrode may be secured to the crystal in the same manner as that shown in Figs. 3 and 6. Another possibility of securing the electrode consists in metallizing the crystal surface and then in securing the electrode to this metallic coating by soldering. The metallic coating is indicated in Fig. 7 by the numeral IS. The coating and the grid form in this embodiment the electrode. The grid soldered to the coating stifiens' and reinforces the latter.

What is claimed is:

l. A piezo-electric crystal oscillator for pro- I crystal being in contact with the liquid to be treated, and means for inhibiting the radiation of energy from one side of said crystal.

2. A piezo-electric crystal oscillator for producing oscillations in liquids, said oscillator comprising a crystal having a vibrating surface which is in contact with the liquid to be treated, and an electrode which is in engagementwith said vi brating surface, said electrode comprising means forming an open-meshed grid structure projecting from said vibrating surface into said liquid.

3. The oscillator defined in claim 2, wherein said electrode is held in engagement with said vibrating surface by means which resiliently contact the periphery thereof.

4. The oscillator defined in claim 2, wherein the depth of said grid is at least approximately one-half of the wave length or an integral multiple thereof.

5. The oscillator defined in claim 2, together with means forming a grid structure positioned in engagement with the opposite vibrating surface of said crystal and projecting from said surface, andmeans disposed adjacent said last noted grid structure for inhibiting the radiation of energy from the corresponding vibrating surface of said crystal.

6. The oscillator defined in claim 2, together with a relatively thin perforate plate which is firmly attached to said vibrating surface, said grid structure projecting from said plate,

'7. The oscillator defined in claim 2, together with a'perforate plate the thickness of which is less than one-half of the wave length and which is firmly attached to said vibrating surface, said grid structure projecting from said plate, the total depth of said plate and saidgrid structure being approximately one-half of the wave length or an integral multiple thereof.

8. The oscillator defined in claim 2, wherein said vibrating surface is metallized, together with a perforate plate soldered to-said metallized surface and carrying said grid structure.

9. The oscillator defined in claim 2', together with means in said electrode forming passages for receiving a cooling medium.

10. A piezo-electric crystal oscillator comprising a plate-shapedcrystalforming two oppositely disposed parallel vibrating surfaces, an electrode for each vibrating surface, each said electrode lation thereto and adjacent the electrode struc- 

